The present invention relates to metering pumps for pumping precise amounts of fluids and, more particularly, to a pneumatically driven, multi-diaphragm fluid injection metering pump.
It is known to use displacement pumps for high pressure chemical injection applications. These pumps may be driven and controlled pneumatically whereby pressurized air, or some other fluid, is pulsed intermittently to a power unit of the pump, which typically comprises driving a piston through a cylinder. The pneumatic controller may be set to pulse at any rate within the pump's operating range. For instance, if controlled from a metering device in the discharge flow line, the pneumatic controller triggers the fluid supply to the pump power unit at a rate proportional to said flow line. A pneumatic controller for injection pumps is shown in U.S. Pat. No. 3,387,563, issued Jun. 11, 1968.
In a known multi-stage pump, a drive piston in a "motor" chamber, moves a pumping piston of smaller diameter in an adjacent "pumping" chamber, which in turn displaces a pumping diaphragm. The differential between the surface area of the drive piston over that of the pumping piston effects a proportional increase in pressure supplied to the pumping chamber. To illustrate, suppose in a particular embodiment of a multi-stage pump, the surface area of the drive piston is A in.sup.2, and the surface area of the pumping piston is A/4 in.sup.2, thereby providing a 4:1 power ratio. For an input pressure of B psi. from the controller, the force exerted on the pumping piston would B*A lbs. Thus, the pressure within the pumping chamber due to the pumping piston would be A*B.div.A/4=4B psi. Thus, a multi-stage configuration is useful where the source of the pressurization driving the pump is not sufficient to overcome the pressure in the discharge line.
Pumping diaphragm failures can occur in high pressure applications because of the difference in pressure on the "pumping" side of the diaphragm, i.e., that side adjacent the pumping intake and discharge chamber, versus the "drive" side of said diaphragm, i.e., that side adjacent the pumping piston. This pressure differential is magnified as a pumping stroke takes place and the pressure exerted by the pumping piston on the middle area of the drive side of the diaphragm causes the transient fluid located on the pumping side to exert a counteracting force upon the outer circumference of the pumping side, which, over time, can cause ruptures. Further, because of the flexible properties inherent in a diaphragm, energy translated by the drive and pumping pistons, respectively, to the pumping diaphragm and exerted on the transient fluid is wasted "pushing" the transient fluid back against the outer circumference of the pumping side of the diaphragm, instead of out the discharge line.
The drive piston must be effectively sealed within the motor chamber pump housing in order to prevent the pressurized fluid from "escaping" around the piston and into the interior of the pump housing, thereby weakening the force the fluid exerts on the piston and causing the pump to fail. However, because of the constant movement of the drive piston, seals for this type of application are subject to heavy wear and failure, requiring that the pump be shut down frequently for repairs.
The present invention is directed at providing an improved multi-stage pump that does not require motor chamber piston seals and has means for equalizing the pressure exerted on both sides of the pumping diaphragm, thereby reducing the susceptibility of the pump to seal and diaphragm failures.